A variable force archwire made from a nickel-titanium (Ni-Ti) alloy wire is shown in Japanese utility model publication SHOO 63-34650 (1988). Different sections of the archwire are treated to develop different moduli of elasticity, thus imparting different forces when deflected. Typically, the sections corresponding to the front teeth, the bicuspids, and the molars are heat treated differently so that each section exhibits a unique modulus of elasticity. As a result, when the archwire is mounted to a patient's teeth, it imparts different forces to different sections of the dental arch.
A typical variable force archwire is heat treated so that its section corresponding to the molars imparts a greater force when deflected than its section corresponding to the bicuspids. The section corresponding to the bicuspids is typically heat treated to impart a greater force when deflected than the section corresponding to the front teeth.
One problem with non-variable force archwires is that the entire length of such an archwire exerts substantially the same force when deflected. Therefore, because it is necessary to impart different forces to different sections of a patient's dental arch, more than one archwire must be employed during the course of treatment. Typically, 7 to 8 different archwires may be used to treat a single patient. With variable force archwires, however, fewer archwires are required.
In FIG. 1, the teeth T of a typical dental arch are illustrated. An orthodontic archwire W is ligated to several orthodontic brackets B mounted to the teeth T. The bending and resultant tension within the archwire W creates forces that are imparted to the brackets B and thus the teeth T. The distal ends WA of the archwire W are secured to the brackets B mounted to the anterior molars, to prevent the archwire from slipping in the mesial direction.
With known stainless steel orthodontic archwires, the distal ends are typically bent into appropriate shapes to prevent them from slipping through the orthodontic brackets B. Typical stainless steel archwires are illustrated in FIGS. 2 and 3. As shown in FIG. 2, the distal end WA of the archwire W is formed into a triangular shape. In FIG. 3, the distal end WA is formed into a hook shape. The bent distal ends are often referred to as "stops".
One problem with shape-memory alloy archwires, such as Ni-Ti archwires, is that it has not been possible to bend their distal ends to form stops. Because of the properties of shape-memory alloy wires, when their distal ends are bent to form stops, the bent shapes are not retained because the archwires tend to return to their original shapes. It has been necessary therefore to attach fastening accessories to their distal ends to prevent them from slipping through the orthodontic appliances. For example, stop tubes are typically mechanically fastened to the distal ends of shape-memory alloy archwires. The procedure of attaching fastening accessories, however, has proven to be relatively complicated and time consuming.
It is an object of the present invention, therefore, to overcome the problems and disadvantages of known shape-memory alloy archwires.